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authorJeff King <peff@peff.net>2019-09-12 14:44:45 (GMT)
committerJunio C Hamano <gitster@pobox.com>2019-09-12 19:30:08 (GMT)
commit6abada1880156df9c6a0de91fb23716e378e13d7 (patch)
tree9de02f0dea7a699bb7e6e16301ec71a84e507f4b /upload-pack.c
parentfbab552a53a2558ec84a06034e6657fb95227266 (diff)
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upload-pack: disable commit graph more gently for shallow traversal
When the client has asked for certain shallow options like "deepen-since", we do a custom rev-list walk that pretends to be shallow. Before doing so, we have to disable the commit-graph, since it is not compatible with the shallow view of the repository. That's handled by 829a321569 (commit-graph: close_commit_graph before shallow walk, 2018-08-20). That commit literally closes and frees our repo->objects->commit_graph struct. That creates an interesting problem for commits that have _already_ been parsed using the commit graph. Their commit->object.parsed flag is set, their commit->graph_pos is set, but their commit->maybe_tree may still be NULL. When somebody later calls repo_get_commit_tree(), we see that we haven't loaded the tree oid yet and try to get it from the commit graph. But since it has been freed, we segfault! So the root of the issue is a data dependency between the commit's lazy-load of the tree oid and the fact that the commit graph can go away mid-process. How can we resolve it? There are a couple of general approaches: 1. The obvious answer is to avoid loading the tree from the graph when we see that it's NULL. But then what do we return for the tree oid? If we return NULL, our caller in do_traverse() will rightly complain that we have no tree. We'd have to fallback to loading the actual commit object and re-parsing it. That requires teaching parse_commit_buffer() to understand re-parsing (i.e., not starting from a clean slate and not leaking any allocated bits like parent list pointers). 2. When we close the commit graph, walk through the set of in-memory objects and clear any graph_pos pointers. But this means we also have to "unparse" any such commits so that we know they still need to open the commit object to fill in their trees. So it's no less complicated than (1), and is more expensive (since we clear objects we might not later need). 3. Stop freeing the commit-graph struct. Continue to let it be used for lazy-loads of tree oids, but let upload-pack specify that it shouldn't be used for further commit parsing. 4. Push the whole shallow rev-list out to its own sub-process, with the commit-graph disabled from the start, giving it a clean memory space to work from. I've chosen (3) here. Options (1) and (2) would work, but are non-trivial to implement. Option (4) is more expensive, and I'm not sure how complicated it is (shelling out for the actual rev-list part is easy, but we do then parse the resulting commits internally, and I'm not clear which parts need to be handling shallow-ness). The new test in t5500 triggers this segfault, but see the comments there for how horribly intimate it has to be with how both upload-pack and commit graphs work. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'upload-pack.c')
-rw-r--r--upload-pack.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/upload-pack.c b/upload-pack.c
index 222cd3a..135bb3f 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -722,7 +722,7 @@ static void deepen_by_rev_list(struct packet_writer *writer, int ac,
{
struct commit_list *result;
- close_commit_graph(the_repository->objects);
+ disable_commit_graph(the_repository);
result = get_shallow_commits_by_rev_list(ac, av, SHALLOW, NOT_SHALLOW);
send_shallow(writer, result);
free_commit_list(result);